N,S-Doped hollow carbon nanosheet-encapsulated Co 9 S 8 nanoparticles as a highly efficient bifunctional electrocatalyst for rechargeable zinc-air batteries.

The development of bifunctional electrocatalysts based on non-noble metals for the oxygen reduction/evolution reactions (ORR/OER) that have rationally designed structures and inexpensive components is of practical significance for the commercialization of rechargeable zinc-air batteries. Here, we report the rational synthesis of Co 9 S 8 nanoparticles embedded in N,S co-doped hollow carbon nanosheets (Co 9 S 8 /NSC) as highly efficient oxygen electrocatalysts. The catalyst is formed when a Co-based zeolitic imidazolate framework (ZIF-67), grown on a Zn-based ZIF (ZIF-8) template, is partially vulcanized following thioacetamide (TAA) and thermal treatment. The resulting catalyst, Co 9 S 8 /NSC-3, shows satisfactory bifunctional electrocatalytic activity in 0.1 M KOH, in which the half-wave potential ( E 1/2 ) for the ORR is 0.82 V and the overpotential for the OER at 10 mA cm -2 is just 350 mV. Furthermore, as the air electrode material in a practical demonstration of a rechargeable liquid zinc-air battery, Co 9 S 8 /NSC-3 exhibits promising battery performance with a high specific capacity of 804 mA h g -1 and a pleasing charge/discharge cyclability of over 140 h at 10 mA cm -2 . The satisfactory activity of Co 9 S 8 /NSC-3 can be attributed to the synergistic effect of the Co 9 S 8 nanoparticles with the N,S-doped hollow carbon nanosheet structure, resulting in an effective electrochemically active surface with fully exposed active sites that give fast catalytic kinetics.